Buffered thin module system and method

ABSTRACT

Multiple fully buffered DIMM circuits or instantiations are presented in a single module. In a preferred embodiment, memory integrated circuits (preferably CSPs) and accompanying AMBs are arranged in two ranks in two fields on each side of a flexible circuit. The flexible circuit has expansion contacts disposed along one side. The flexible circuit is disposed about a supporting substrate or board to place one complete FB-DIMM circuit or instantiation on each side of the constructed module. In alternative but also preferred embodiments, the ICs on the side of the flexible circuit closest to the substrate are disposed, at least partially, in what are, in a preferred embodiment, windows, pockets, or cutaway areas in the substrate. Other embodiments may only populate one side of the flexible circuit or may only remove enough substrate material to reduce but not eliminate the entire substrate contribution to overall profile. The flexible circuit may exhibit one or two or more conductive layers, and may have changes in the layered structure or have split layers. Other embodiments may stagger or offset the ICs or include greater numbers of ICs.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/934,027, filed Sep. 3, 2004. U.S. patent application Ser.No. 10/934,027 has been incorporated by reference herein.

FIELD

The present invention relates to systems and methods for creating highdensity circuit modules.

BACKGROUND

The well-known DIMM (Dual In-line Memory Module) board has been used foryears, in various forms, to provide memory expansion. A typical DIMMincludes a conventional PCB (printed circuit board) with memory devicesand supporting digital logic devices mounted on both sides. The DIMM istypically mounted in the host computer system by inserting acontact-bearing edge of the DIMM into a card edge connector. Systemsthat employ DIMMs provide, however, very limited profile space for suchdevices and conventional DIMM-based solutions have typically providedonly a moderate amount of memory expansion.

As bus speeds have increased, fewer devices per channel can be reliablyaddressed with a DIMM-based solution. For example, 288 ICs or devicesper channel may be addressed using the SDRAM-100 bus protocol with anunbuffered DIMM. Using the DDR-200 bus protocol, approximately 144devices may be address per channel. With the DDR2-400 bus protocol, only72 devices per channel may be addressed. This constraint has led to thedevelopment of the fully-buffered DIMM (FB-DIMM) with buffered C/A anddata in which 288 devices per channel may be addressed. With theFB-DIMM, not only has capacity increased, pin count has declined toapproximately 69 from the approximately 240 pins previously required.

The FB-DIMM circuit solution is expected to offer practical motherboardmemory capacities of up to about 192 gigabytes with six channels andeight DIMMs per channel and two ranks per DIMM using one gigabyte DRAMs.This solution should also be adaptable to next generation technologiesand should exhibit significant downward compatibility.

This great has, however, come with some cost and will eventually beself-limiting. The basic principle of systems that employ FB-DIMM reliesupon a point-to-point or serial addressing scheme rather than theparallel multi-drop interface that dictates non-buffered DIMMaddressing. That is, one DIMM is in point-to-point relationship with thememory controller and each DIMM is in point-to-point relationship withadjacent DIMMs. Consequently, as bus speeds increase, the number of DIMson a bus will decline as the discontinuities caused by the chain ofpoint to point connections from the controller to the “last” DIMM becomemagnified in effect as speeds increase. Consequently, methods toincrease the capacity of a single DIMM find value in contemporary memoryand computing systems.

There are several known methods to improve the limited capacity of aDIMM or other circuit board. In one strategy, for example, small circuitboards (daughter cards) are connected to the DIMM to provide extramounting space. The additional connection may cause, however, flawedsignal integrity for the data signals passing from the DIMM to thedaughter card and the additional thickness of the daughter card(s)increases the profile of the DIMM.

Multiple die packages (MDP) are also used to increase DIMM capacitywhile preserving profile conformity. This scheme increases the capacityof the memory devices on the DIMM by including multiple semiconductordie in a single device package. The additional heat generated by themultiple die typically requires, however, additional coolingcapabilities to operate at maximum operating speed. Further, the MDPscheme may exhibit increased costs because of increased yield loss frompackaging together multiple die that are not fully pre-tested.

Stacked packages are yet another strategy used to increase circuit boardcapacity. This scheme increases capacity by stacking packaged integratedcircuits to create a high-density circuit module for mounting on thecircuit board. In some techniques, flexible conductors are used toselectively interconnect packaged integrated circuits. Staktek GroupL.P. has developed numerous systems for aggregating CSP (chipscalepackaged) devices in space saving topologies. The increased componentheight of some stacking techniques may alter, however, systemrequirements such as, for example, required cooling airflow or theminimum spacing around a circuit board on its host system.

What is needed, however, are methods and structures for increasing theflexibility of the FB-DIMM solution.

SUMMARY

Multiple fully buffered DIMM circuits or instantiations are combined ina single module to provide on a single module circuitry that issubstantially the functional equivalent of two or more FB-DIMMs butavoids some of the drawbacks associated with having two discreteFB-DIMMs. In a preferred embodiment, integrated circuits (preferablymemory CSPs) and accompanying AMBs are arranged in two ranks in twofields on each side of a flexible circuit. The flexible circuit hasexpansion contacts disposed along one side. The flexible circuit isdisposed about a supporting substrate or board to place at least oneFB-DIMM instantiation on each side of the constructed module. Inalternative, but also preferred embodiments, the ICs on the side of theflexible circuit closest to the substrate are disposed, at leastpartially, in what are, in a preferred embodiment, windows, pockets, orcutaway areas in the substrate. Other embodiments may only populate oneside of the flexible circuit or may only remove enough substratematerial to reduce but not eliminate the entire substrate contributionto overall profile. Other embodiments may connect the constituentdevices in a way that creates a FB-DIMM circuit or instantiation withthe devices on the upper half of the module while another FB-DIMMinstantiation is created with the devices on the lower half of themodule. Other embodiments may, for example, combine selected circuitryfrom one side of the module (memory CSPs for example) with circuitry onthe other side of the module (an AMB, for example) in creating one ofplural FB-DIMM instantiations on a single module. Other embodimentsemploy stacks to provide multiple FB-DIMM circuits or instantiations ona low profile module. The flexible circuit may exhibit one or two ormore conductive layers, and may have changes in the layered structure orhave split layers. Other embodiments may stagger or offset the ICs orinclude greater numbers of ICs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a depiction of a preferred embodiment of a module devised inaccordance with the present invention.

FIG. 2 depicts a contact bearing first side of a flex circuit devised inaccordance with a preferred embodiment of the present invention.

FIG. 3 depicts the second side of the exemplar populated flex circuit ofFIG. 2.

FIG. 4 is a cross-sectional depiction through the devices as populatedin an embodiment of the present invention.

FIG. 5 is an enlarged view of the area marked ‘A’ in FIG. 4.

FIG. 6 depicts a cross-sectional view of a module devised in accordancewith an alternate preferred embodiment of the present invention.

FIG. 7 depicts the area near an end of a substrate in the embodimentshown in FIG. 6.

FIG. 8 depicts a cross-sectional view of a module assembly devised inaccordance with a preferred embodiment of the present invention.

FIG. 9 is an enlarged view of a portion of one preferred embodiment.

FIG. 10 depicts one perspective of an exemplar module devised inaccordance with a preferred embodiment of the present invention.

FIG. 11 is another depiction of the relationship between flex circuitryand a substrate 14 which has been patterned or windowed with cutawayareas.

FIG. 12 depicts a cross sectional view of an exemplar substrate employedin FIG. 11 before being combined with populated flex circuits.

FIG. 13 depicts another embodiment of the invention having additionalICs.

FIG. 14 is a representation of impedance discontinuities in typicalFB-DIMM systems.

FIG. 15 is a representation of impedance discontinuities in anembodiment of the present invention.

FIG. 16 depicts yet another embodiment of the present invention.

FIG. 17 presents another embodiment of the present invention.

FIG. 18 depicts a low profile embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 depicts a preferred embodiment devised in accordance with thepresent invention. Module 10 is depicted in FIG. 1 exhibiting ICs 18 andcircuit 19.

FIG. 2 depicts a first side 8 of flex circuit 12 (“flex”, “flexcircuitry”, “flexible circuit”) used in constructing a module accordingto an embodiment of the present invention. Flex circuit 12 is preferablymade from one or more conductive layers supported by one or moreflexible substrate layers as further described with reference to laterFigs. The construction of flex circuitry is known in the art. Theentirety of the flex circuit 12 may be flexible or, as those of skill inthe art will recognize, the flexible circuit structure 12 may be madeflexible in certain areas to allow conformability to required shapes orbends, and rigid in other areas to provide rigid and planar mountingsurfaces. Preferred flex circuit 12 has openings 17 for use in aligningflex circuit 12 to substrate 14 during assembly.

ICs 18 on flexible circuit 12 are, in this embodiment, chip-scalepackaged memory devices of small scale. For purposes of this disclosure,the term chip-scale or “CSP” shall refer to integrated circuitry of anyfunction with an array package providing connection to one or more diethrough contacts (often embodied as “bumps” or “balls” for example)distributed across a major surface of the package or die. CSP does notrefer to leaded devices that provide connection to an integrated circuitwithin the package through leads emergent from at least one side of theperiphery of the package such as, for example, a TSOP.

Embodiments of the present invention may be employed with leaded or CSPdevices or other devices in both packaged and unpackaged forms but wherethe term CSP is used, the above definition for CSP should be adopted.Consequently, although CSP excludes leaded devices, references to CSPare to be broadly construed to include the large variety of arraydevices (and not to be limited to memory only) and whether die-sized orother size such as BGA and micro BGA as well as flip-chip. As those ofskill will understand after appreciating this disclosure, someembodiments of the present invention may be devised to employ stacks ofICs each disposed where an IC 18 is indicated in the exemplar Figs.

Multiple integrated circuit die may be included in a package depicted asa single IC 18. While in this embodiment memory ICs are used to providea memory expansion board or module, and various embodiments may includea variety of integrated circuits and other components. Such variety mayinclude microprocessors, FPGA's, RF transceiver circuitry, digitallogic, as a list of non-limiting examples, or other circuits or systemswhich may benefit from a high-density circuit board or modulecapability. Circuit 19 depicted between ICs 18 may be a memory buffer orcontroller but in a preferred embodiment is the well known advancedmemory buffer or “AMB”.

The depiction of FIG. 2 shows flex circuit 12 as having first and secondfields F1 and F2. Each of fields F1 and F2 have at least one mountingcontact array for CSPs such as the one depicted by reference 11A.Contact arrays such as array 11 are disposed beneath ICs 18 and circuits19. An exemplar contact array 11A is shown as is exemplar IC 18 to bemounted at contact array 11A as depicted. The contact arrays 11A thatcorrespond to an IC plurality may be considered a contact array set.

Field F1 of side 8 of flex circuit 12 is shown populated with firstplurality of CSPs IC_(R1) and second plurality of CSPs IC_(R2) whilesecond field F2 of side 8 of flex circuit 12 is shown populated withfirst plurality of CSPs IC_(R1) and second plurality of CSPs IC_(R2).Those of skill will recognize that the identified pluralities of CSPsare, when disposed in the configurations depicted, typically describedas “ranks”. Between the ranks IC_(R2) of field F1 and IC_(R2) of fieldF2, flex circuit 12 bears a plurality of module contacts allocated inthis embodiment into two rows (C_(R1) and C_(R2)) of module contacts 20.When flex circuit 12 is folded as later depicted, side 8 depicted inFIG. 2 is presented at the outside of module 10. The opposing side 9 offlex circuit 12 is on the inside in several depicted configurations ofmodule 10 and thus side 9 is closer to the substrate 14 about which flexcircuit 12 is disposed than is side 8. Other embodiments may have othernumbers of ranks and combinations of plural CSPs connected to create themodule of the present invention.

FIG. 3 shows side 9 of flex circuit 12 depicting the other side of theflex circuit shown in FIG. 2. Side 9 of flex circuit 12 is shown asbeing populated with multiple CSPs 18. Side 9 includes fields F1 and F2that each include at least one mounting contact array site for CSPs and,in the depicted case, include multiple contact arrays. Each of fields F1and F2 include, in the depicted preferred embodiment, two pluralities ofICs identified in FIG. 3 as IC_(R1) and IC_(R2). Thus, each side of flexcircuit 12 has, in a preferred embodiment, two fields F1 and F2 each ofwhich fields includes two ranks of CSPs IC_(R1) and IC_(R2). In laterFIG. 4, it will be recognized that fields F1 and F2 will be disposed ondifferent sides of substrate 14 in a completed module 10 when ICs 18 areidentified according to the organizational identification depicted inFIGS. 2 and 3 but those of skill will recognize that the groupings ofICs 18 shown in FIGS. 2 and 3 are not dictated by the invention but areprovided merely as an exemplar organizational strategy to assist inunderstanding the present invention.

Various discrete components such as termination resistors, bypasscapacitors, and bias resistors, in addition to the buffers 19 shown onside 8 of flex circuit 12, may be mounted on either or both of sides 8and 9 of flex 12. Such discrete components are not shown to simplify thedrawing. Flex circuit 12 may also depicted with reference to itsperimeter edges, two of which are typically long (PE_(long1) andPE_(long 2)) and two of which are typically shorter (PE_(short1) andPE_(short2)). Other embodiments may employ flex circuits 12 that are notrectangular in shape and may be square in which case the perimeter edgeswould be of equal size or other convenient shape to adapt tomanufacturing particulars. Other embodiments may also have fewer orgreater numbers of ranks or pluralities of ICs in each field or on aside of a flex circuit.

FIG. 2 depicts an exemplar conductive trace 21 connecting row C_(R2) ofmodule contacts 20 to ICs 18. Those of skill will understand that thereare many such traces in a typical embodiment. Traces 21 may also connectto vias that may transit to other conductive layers of flex 12 incertain embodiments having more than one conductive layer. In apreferred embodiment, vias connect ICs 18 on side 9 of flex 12 to modulecontacts 20. An example via is shown as reference 23. Traces 21 may makeother connections between the ICs on either side of flex 12 and maytraverse the rows of module contacts 20 to interconnect ICs. Togetherthe various traces and vias make interconnections needed to convey dataand control signals amongst the various ICs and buffer circuits. Thoseof skill will understand that the present invention may be implementedwith only a single row of module contacts 20 and may, in otherembodiments, be implemented as a module bearing ICs on only one side offlex circuit 12.

FIG. 4 is a cross section view of a module 10 devised in accordance witha preferred embodiment of the present invention. Module 10 is populatedwith ICs 18 having top surfaces 18 _(T) and bottom surfaces 18 _(B).Substrate or support structure 14 has first and second perimeter edges16A and 16B appearing in the depiction of FIG. 4 as ends. Substrate orsupport structure 14 typically has first and second lateral sides S₁ andS₂. Flex 12 is wrapped about perimeter edge 16A of substrate 14, whichin the depicted embodiment, provides the basic shape of a common DIMMboard form factor such as that defined by JEDEC standard MO-256.

FIG. 5 is an enlarged view of the area marked ‘A’ in FIG. 4. Edge 16A ofsubstrate 14 is shaped like a male side edge of an edge card connector.While a particular oval-like configuration is shown, edge 16A may takeon other shapes devised to mate with various connectors or sockets. Theform and function of various edge card connectors are well know in theart. In many preferred embodiments, flex 12 is wrapped around edge 16Aof substrate 14 and may be laminated or adhesively connected tosubstrate 14 with adhesive 30. The depicted adhesive 30 and flex 12 mayvary in thickness and are not drawn to scale to simplify the drawing.The depicted substrate 14 has a thickness such that when assembled withthe flex 12 and adhesive 30, the thickness measured between modulecontacts 20 falls in the range specified for the mating connector. Insome other embodiments, flex circuit 12 may be wrapped about perimeteredge 16B or both perimeter edges 16A and 16B of substrate 14. In otherinstances, multiple flex circuits may be employed or a single flexcircuit may connect one or both sets of contacts 20 to the resident ICs.

FIG. 6 depicts a cross-sectional view of a module 10 devised inaccordance with an alternate preferred embodiment of the presentinvention with the view taken along a line through two AMBs and selectedICs 18 from IC_(R1). The module 10 depicted in FIG. 6 differs from thatshown in earlier embodiments in that rather than a single flex circuit12, the depicted exemplar module 10 employs two flex circuits 12A and12B with 12A being disposed on one lateral side S1 of substrate 14 whileflex circuit 12B is employed on lateral side S2 of substrate 14.

FIG. 7 depicts the area near end 16A of substrate 14 in the embodimentshown in FIG. 6 that employs two flex circuits identified as 12A and 12Bto implement a module in accordance with an alternate preferredembodiment of the present invention. Each of flex circuits 12A and 12Bare populated with ICs 18 on one or both of their respective sides 8 and9 and each of flex circuits 12A and 12B employ a buffer circuit 19 suchas, for example, an advanced buffer circuit or AMB to implement, alongwith the resident CSPs, multiple FB-DIMM circuits mounted on a singlemodule 10. The area on side 9 of each of flex circuits 12A and 12Bopposite the disposition of buffer circuit 19 disposed along side 8 offlex circuits 12A and 12B is, in the depicted module, filled with aconformal material 31 to provide support along the length of module 10where structure is not provided by the bodies of circuits such as ICs 18or buffers 19.

FIG. 8 depicts a cross-sectional view of a module 10 devised with asubstrate 14 that has cutaway areas into which ICs 18 are disposed toreduce the profile of module 10. Corresponding ICs 18 from each offields F1 and F2 pass through windows 250 in substrate 14 as shown inlater Figs. in further detail and the inner ICs 18 are preferablyattached to each other's upper surfaces 18 _(T) with a thermallyconductive adhesive 30. While in this embodiment, the depicted ICs areattached to flex circuit 12 in opposing pairs, fewer or greater numbersof ICs may be connected in other arrangements such as, for example,staggered or offset arrangements in which they may exhibit preferredthermal characteristics. In a preferred embodiment, ICs 18 will be CSPsand typically, memory CSPs. To simplify the drawing, discrete componentssuch as resistors and capacitors typically found on embodiments ofmodule 10 are not shown.

In this embodiment, flex circuit 12 has module contacts 20 positioned ina manner devised to fit in a circuit board card edge connector or socketand connect to corresponding contacts in the connector (not shown).While module contacts 20 are shown protruding from the surface of flexcircuit 12, other embodiments may have flush contacts or contacts belowthe surface level of flex 12. Substrate 14 supports module contacts 20from behind flex circuit 12 in a manner devised to provide themechanical form required for insertion into a socket. In otherembodiments, the thickness or shape of substrate 14 in the vicinity ofperimeter edge 16A may differ from that in the vicinity of perimeteredge 16B. Substrate 14 in the depicted embodiment is preferably made ofa metal such as aluminum or copper, as non-limiting examples, or wherethermal management is less of an issue, materials such as FR4 (flameretardant type 4) epoxy laminate, PTFE (poly-tetra-fluoro-ethylene) orplastic. In another embodiment, advantageous features from multipletechnologies may be combined with use of FR4 having a layer of copper onboth sides to provide a substrate 14 devised from familiar materialswhich may provide heat conduction or a ground plane.

FIG. 9 is an enlarged view of a portion of one preferred embodimentshowing lower IC 18 ₁ and upper IC 18 ₂ and substrate 14 having cutawayareas into which ICs 18 are disposed. In this embodiment, conductivelayer 66 of flex circuit 12 contains conductive traces connecting modulecontacts 20 to BGA contacts 63 on ICs 18 ₁ and 18 ₂. The number oflayers may be devised in a manner to achieve the bend radius required inthose embodiments that bend flex circuit 12 around edge 16A or 16B, forexample. The number of layers in any particular portion of flex circuit12 may also be devised to achieve the necessary connection density givena particular minimum trace width associated with the flex circuittechnology used. Some flex circuits 12 may have three or four or moreconductive layers. Such layers may be beneficial to route signals in aFB-DIMM which may have fewer DIMM input/output signals than a registeredDIMM, but may have more interconnect traces required among devices onthe DIMM, such as, for example, the C/A copy A and C/A copy B(command/address) signals produced by an FB-DIMM AMB.

In this embodiment, there are three layers of flex circuit 12 betweenthe two depicted ICs 18 ₁ and 18 ₂. Conductive layers 64 and 66 expressconductive traces that connect to the ICs and may further connect toother discrete components (not shown). Preferably, the conductive layersare metal such as, for example, copper or alloy 110. Vias such as theexemplar vias 23 connect the two conductive layers 64 and 66 and therebyenable connection between conductive layer 64 and module contacts 20. Inthis preferred embodiment having a three-layer portion of flex circuit12, the two conductive layers 64 and 66 may be devised in a manner sothat one of them has substantial area employed as a ground plane. Theother layer may employ substantial area as a voltage reference plane.The use of plural conductive layers provides advantages and the creationof a distributed capacitance intended to reduce noise or bounce effectsthat can, particularly at higher frequencies, degrade signal integrity,as those of skill in the art will recognize. If more than two conductivelayers are employed, additional conductive layers may be added withinsulating layers separating conductive layers. Portions of flex circuit12 may in some embodiments be rigid portions (rigid-flex). Constructionof rigid-flex circuitry is known in the art.

With the construction of an embodiment such as that shown in FIG. 9,thermal energy will be urged to move between the respective ICs 18 ₁.Thus, the ICs become a thermal mass sharing the thermal load. Flexcircuit 12 may be particularly devised to operate as a heat spreader orsink adding to the thermal conduction out of ICs 18 ₁ and 18 ₂.

FIG. 10 depicts one perspective of an exemplar module 10 devised inaccordance with a preferred embodiment of the present invention. Asthose of skill will understand, the depiction of FIG. 10 is simplifiedto show more clearly the principles of the invention but depicts fewerICs 18 than would typically be presented in embodiments of the presentinvention.

The principles of the present invention may, however, be employed whereonly one IC 18 is resident on a side of a flex circuit 12 or wheremultiple ranks or pluralities of ICS are resident on a side of flexcircuit 12, or, as will be later shown, where multiple ICs 18 aredisposed one atop the other to give a single module 10 materiallygreater.

FIG. 10 depicts a cross sectional view of an exemplar module showing areduced number of ICs 18 to allow a clearer exposition of the principlesof the present invention as illustrated by this depicted embodiment. Themodule shown in FIG. 10 is formed with an exemplar flex circuit such asthat depicted in FIGS. 2 and 3. The second side 9 of flex circuit 12shown in FIG. 3 is folded about substrate 14 shown in FIG. 10 to placeICs 18 into the windows 250 arrayed along substrate 14. This results inICs of ranks IC_(R1) and IC_(R2) being disposed back to back withinwindows 250. Preferably, a thermally conductive adhesive or glue is usedon the upper sides of ICs 18 to encourage thermal energy flow as well asprovide some mechanical advantages. Those of skill will recognize thatin this embodiment, where FIG. 10 depicts the first or, in this case,the outer side of the flex circuit once combined with substrate 14, theflex circuit itself will have staggered mounting arrays 11A on side 8 offlex circuit 12 relative to side 9 of flex circuit 12. This is merelyone relative arrangement between ICs 18 on respective sides of substrate14.

As shown in FIG. 10, ICs 18 which are on second side 9 (which in thisdepiction is the inner side with respect to the module 10) of populatedflex circuit 12 are disposed in windows 250 so that the upper surfaces18 _(T) of ICs 18 of row ICR1 of F1 are in close proximity with theupper surfaces 18 _(T) of ICs 18 of rank ICR1 of F2. Thus, a first groupof ICs (CSPs in the depiction) may be considered to be comprised of theICs of IC_(R1) from both fields F1 and F2 while a second group of ICsmay be considered to be comprised of the ICs of IC_(R2) from both fieldsF1 and F2. The ICs 18 that are populated along side 9 of flex circuit 12are positioned in the cutaway areas of the first and second lateralsides, respectively, of substrate 14. In this case, the cutaway areas oneach lateral side of substrate 14 are in spatial coincidence to createwindows 250. Those of skill will recognize that the depiction is not toscale but representative of the interrelationships between the elementsand the arrangement results in a profile “P” for module 10 that issignificantly smaller than it would have been without fitting ICs 18along inner side 9 of flex circuit 12 into windows 250. Profile P inthis case is approximately the sum of the distances between the upperand lower surfaces of IC 18 plus 4× the diameter of the BGA contacts 63plus 2× the thickness of flex circuit 12 in addition to any adhesivelayers 30 employed to adhere one IC 18 to another. This profiledimension will vary depending upon whether BGA contacts 63 are disposedbelow the surface of flex circuit 12 to reach an appropriate conductivelayer or contacts which typically are a part of flex circuit 12.

FIG. 11 is another depiction of the relationship between flex circuitryand a substrate 14 which has been patterned or windowed with cutawayareas. The view of FIG. 11 is taken along a line that would intersectthe bodies of ICs 18. In FIG. 11, two flex circuits 12A and 12B areshown populated along their respective sides 9 with ICs 18 (i.e., CSPsin the depiction). The ICs 18 along the inner side 9 of flex circuit 12Aare staggered relative to those that are along inner side 9 of flexcircuit 12B when module 10 is assembled and flex circuits 12A and 12Bare combined with substrate 14. This staggering may result in someconstruction benefits providing a mechanical “step” for ICs 18 as theyare fitted into substrate 14 and may further provide some thermaladvantages increasing the contact area between substrate 14 and theplurality of ICs 18. Those of skill will recognize that flex circuits12A and 12B even though depicted as being populated on only one side,may be populated on either or both sides 8 and 9 just as in thoseembodiments that employ a single flex circuit 12 may be populated one orboth sides of flex circuit 12 and may have populated one or both fieldsor ranks within fields on one or both sides with CSPs or other circuits.

FIG. 12 depicts a cross sectional view of exemplar substrate 14 employedin FIG. 11 before being combined with populated flex circuits 12A and12B as viewed along a line through windows 250 of substrate 14. Asdepicted in FIG. 12, a number of cutaway areas or pockets are delineatedwith dotted lines and identified with references 250B3 and 250B4,respectively. Those areas identified as 250B3 correspond, in thisexample, to the pockets, sites, or cutaway areas on one side ofsubstrate 14 into which ICs 18 from flex circuit 12A will be disposedwhen substrate 14 and flex circuit 12A are combined. Those pocket,sites, or cutaway areas identified as references 250B4 correspond to thesites into which ICs 18 from flex circuit 12B will be disposed.

For purposes herein, the term window may refer to an opening all the waythrough substrate 14 across span “S” which corresponds to the width orheight dimension of packaged IC 18 or, it may also refer to that openingwhere cutaway areas on each of the two sides of substrate 14 overlap.

Where cutaway areas 250B3 and 250B4 overlap, there are, as depicted,windows all the way through substrate 14. In some embodiments, cutawayareas 250B3 and 250B4 may not overlap or in other embodiments, there maybe pockets or cutaway areas only on one side of substrate 14. Those ofskill will recognize that cutaway areas such as those identified withreferences 250B3 and 250B4 may be formed in a variety of ways dependingon the material of substrate 14 and need not literally be “cut” away butmay be formed by a variety of molding, milling and cutting processes asis understood by those in the field.

FIG. 13 depicts another embodiment of the invention having additionalICs 18. In this embodiment, four flex level transitions 26 connect tofour mounting portions 28 of flex circuits 12A₁, 12A₂, 12B₁, and 12B₂.In this embodiment, each mounting portion 28 has ICs 18 on both sides.Flex circuitry 12 may also be provided in this configuration by, forexample, having a split flex with layers interconnected with vias. Asthose of skill will recognize, the possibilities for large capacityiterations of module 10 are magnified by such strategies and the sameprinciples may be employed where the ICs 18 on one side of substrate 14are staggered relative to those ICs 18 on the other side of substrate 14or, substrates such as those shown in FIG. 4 that have no cutaway areasmay be employed.

Four flex circuits are employed in module 10 as depicted in FIG. 13 and,although those embodiments that wrap flex circuit 12 about end 16A ofsubstrate 14 present manufacturing efficiencies, in some environmentshaving flex circuitry separate from each other may be desirable.

In a typical FB-DIMM system employing multiple FB-DIMM circuits, therespective AMB's from one FB-DIMM circuit to another FB-DIMM circuit areseparated by what can be conceived of as three impedance discontinuitiesas represented in the system depicted in FIG. 14 as D1, D2, and D3. FIG.14 includes two modules 10 and includes a representation of theconnection between the two modules. Discontinuity D1 represents theimpedance discontinuity effectuated by the connector-socket combinationassociated with the first module 10F. Discontinuity D2 represents theimpedance perturbation effectuated by the connection between theconnector-socket of first module 10F and the connector-socket of secondmodule 10S while discontinuity D3 represents the discontinuityeffectuated by the connector-socket combination associated with thesecond module 10S. The AMB is the new buffer technology particularly forserver memory and typically includes a number of features includingpass-through logic for reading and writing data and commands andinternal serialization capability, a data bus interface, a deserialingand decode logic capability and clocking functions. The functioning ofan AMB is the principal distinguishing hard feature of a FB-DIMM module.Those of skill will understand how to implement the connections betweenICs 18 and AMB 19 in FB-DIMM circuits implemented by embodiments of thepresent invention and will recognize that the present invention providesadvantages in capacity as well as reduced impedance discontinuity thatcan hinder larger implementations of FB-DIMM systems. Further, those ofskill will recognize that various principles of the present inventioncan be employed to multiple FB-DIMM circuits on a single substrate ormodule.

In contrast to the system represented by FIG. 14, FIG. 15 is a schematicrepresentation of the single impedance perturbation DX effectuated bythe connection between a first AMB 19 of a first FB-DIMM “FB1” of afirst module 10F and a second AMB 19 of a second FB-DIMM “FB2” of thesame first module 10F.

FIG. 16 depicts another embodiment of the present invention in which amodule 10 is devised using stacks to create a module 10 presenting twoFB-DIMM circuits. Those of skill will appreciate that using stacks suchas depicted stacks 40 owned by Staktek Group L.P. allows creatingmodules that have multiple FB-DIMM circuits on a single module. Stacks40 are just one of several stack designs that may be employed with thepresent invention. Stacks 40 are devised with mandrels 42 and stack flexcircuits 44 as described in U.S. patent application Ser. No. 10/453,398,filed Jun. 6, 2003 which is owned by Staktek Group L.P. and which ishereby incorporated by reference and stacks 40 and AMB 19 are mounted onflex circuit 12 which is disposed about substrate 14.

FIG. 17 depicts use of stacks in an embodiment of the present inventionthat exhibits a low profile with use of stacks. Such an embodimentpresents at least two FB-DIMM circuits at its contacts 20.

FIG. 18 illustrates a low profile embodiment of the present invention.The depicted module 10 has at least two AMBs and associated circuitrysuch as ICs 18 which in the preferred mode and as illustrated are CSPsand needed support circuitry to create at least two FB-DIMM circuits orinstantiations on a single module with a low profile. It should beunderstood that the second AMB in addition to the one literally showncan be disposed on either side of module 10 but preferably will bedisposed closer to lateral side S2 of substrate 14 than is the depictedAMB 19 but like AMB 19 will be disposed on side 8 of flex circuit 12. Inthis embodiment, contacts 20 are along side 8 of flex circuit 12 andproximal to edge E of flex circuit 12. The principal circuits thatconstitute the first FB-DIMM circuitry or instantiation (i.e., the CSPsand AMB) may be disposed in single rank file as shown. They may beallocated to first and second mounting fields of the first and secondsides of flex circuit 12 as earlier described with reference to earlierFigs. Those of skill will recognize that contacts 20 may appear on oneor both sides of module 10 depending on the mechanical contactinter-face particulars of the application.

The present invention may be employed to advantage in a variety ofapplications and environment such as, for example, in computers such asservers and notebook computers by being placed in motherboard expansionslots to provide enhanced memory capacity while utilizing fewer sockets.The two high rank embodiments or the single rank high embodiments mayboth be employed to such advantage as those of skill will recognizeafter appreciating this specification.

One advantageous methodology for efficiently assembling a circuit module10 such as described and depicted herein is as follows. In a preferredmethod of assembling a preferred module assembly 10, flex circuit 12 isplaced flat and both sides populated according to circuit board assemblytechniques known in the art. Flex circuit 12 is then folded about end16A of substrate 14. Flex 12 may be laminated or otherwise attached tosubstrate 14.

Although the present invention has been described in detail, it will beapparent to those skilled in the art that many embodiments taking avariety of specific forms and reflecting changes, substitutions andalterations can be made without departing from the spirit and scope ofthe invention. Therefore, the described embodiments illustrate but donot restrict the scope of the claims.

1. A circuit module comprising: a flex circuit having a first side thathas plural edge connector contacts for connection of the circuit moduleto an edge connector, the first side having mounted thereon, a firstadvanced memory buffer (AMB) and plural CSPs (chipscale packageddevices), a second side having mounted thereon, a second advanced memorybuffer (AMB) and plural CSPs; and a rigid substrate having first andsecond opposing lateral Sides and a bottom edge, the Hex circuit beingwrapped about the bottom edge of the rigid substrate to be inserted in asingle edge connector socket, and the second advanced memory buffer andplural CSPs mounted on the second side of the flex circuit beingdisposed proximal to the rigid substrate while the first advanced memorybuffer mad plural CSPs are exposed on the outside of the circuit moduleand the plural edge connector contacts of the flex circuit are supportedby the rigid substrate.
 2. The circuit module of claim 1 connected to acircuit board through the plural edge connector contacts.
 3. The circuitmodule of claim 1 in which at least one of the plural CSPs mounted onthe second side of the flex circuit is adhesively connected to the rigidsubstrate.
 4. The circuit module of claim 3 in which the adhesiveconnection is implemented with a thermally conductive adhesive.
 5. Thecircuit module of claim 1 in which the rigid substrate is comprised ofmetal.
 6. The circuit module of claim 1 in which the rigid substrate iscomprised of aluminum.
 7. The circuit module of claim 1 in which therigid substrate is patterned to provide cutaway areas into which aredisposed one or more of the plural CSPs mounted on the second side ofthe flex circuit.
 8. The circuit module of claim 1 in which the pluralCSPs mounted on the first side of the flex circuit and the firstadvanced memory buffer circuit comprise a FB-DIMM instantiation.
 9. Thecircuit module of claim 1 in which a First group of the plural CSPsmounted on the first side of the flex circuit and a first group of theplural CSPs mounted on the second side of the flex circuit comprise afirst FB-DIMM instantiation.
 10. The circuit module of claim 9 in whicha second group of the plural CSPs mounted on the first side of the flexcircuit and a second group of the plural CSPs mounted on the second sideof the flex circuit comprise a second FB-DIMM instantiation.
 11. Acircuit module comprising: a flex circuit having a first side, and asecond side and along the first side first and second lateralpluralities of edge connector contacts; first and second ranks of CSP(chipscale package) memory devices mounted on the first side of the flexcircuit with the first rank of CSP memory devices being separated fromthe second rank of CSP memory devices by the lateral plurality of edgeconnector contacts; plural CSP memory devices mounted on the second sideof the flex circuit; first and second AMBs (advanced memory buffers)with the first AMB being disposed on the first side of the flex circuitwith the first rank of CSP memory devices and the second AMB beingdisposed on the first side of the flex circuit with the second rank ofCSP memory devices; and a rigid substrate having a first lateral sideand a second lateral side and a bottom edge, the flex circuit beingwrapped about the bottom edge to be inserted in a single edge connectorsocket and the first rank of CSPs being disposed on one external side ofthe circuit module and the second rank of CSPs being disposed on theother external side of the circuit module while the plural CSP memorydevices, mounted on the second side of the flex circuit, are between theflex circuit and the rigid substrate and the first lateral plurality ofedge connector contacts of the flex circuit being supported by the firstlateral side of the rigid substrate and the second lateral plurality ofedge connector contacts of the flex circuit being supported by thesecond lateral side of the rigid substrate.
 12. The circuit module ofclaim 11 in which at least some of the CSP memory devices are comprisedof stacks.
 13. The circuit module of claim 11 in which the substrate iscomprised of metal.
 14. The circuit module of claim 11 set in an edgeconnector socket in a computer.